Obesity is a public health problem that is both serious and widespread. One-third of the population in industrialized countries has an excess weight of at least 20% relative to the ideal weight. The phenomenon continues to worsen, particularly in regions of the globe where economics are modernizing. In the United States, the number of obese people has escalated from 25% at the end of the 70s to 33% at the beginning of the 90s.
Obesity considerably increases the risk of developing cardiovascular or metabolic diseases. It is estimated that if the entire population had an ideal weight, the risk of coronary insufficiency would decrease by 25% and that of cardiac insufficiency and of cerebral vascular accidents by 35%. Coronary insufficiency, atheromatous disease and cardiac insufficiency are at the forefront of the cardiovascular complications induced by obesity. For an excess weight greater than 30%, the incidence of coronary diseases is doubled in subjects under 50 years. Studies carried out for other diseases are equally eloquent. For an excess weight of 20%, the risk of high blood pressure is doubled. For an excess weight of 30%, the risk of developing a non-insulin-dependent diabetes is tripled, and that of hyperlipidemias is multiplied six-fold.
The list of diseases having onsets promoted by obesity includes: hyperuricemia (11.4% in obese subjects, against 3.4% in the general population), digestive pathologies, abnormalities in hepatic functions, and even certain cancers.
Whether the physiological changes in obesity are characterized by an increase in the number of adipose cells, or by an increase in the quantity of triglycerides stored in each adipose cell, or by both, this excess weight results mainly from an imbalance between the quantities of calories consumed and the quantity of calories used by the body. Studies on the causes of this imbalance have been in several directions. Some have focused on studying the mechanism of absorption of foods, and therefore the molecules that control food intake and the feeling of satiety. Other studies have characterized the pathways through which the body uses its calories.
The proposed treatments for obesity are of five types. (1) Food restriction is the most frequently used. The obese individuals are advised to change their dietary habits so as to consume fewer calories. Although this type of treatment is effective in the short-term, the recidivation rate is very high. (2) Increased calorie use through physical exercise is also proposed. This treatment is ineffective when applied alone, but it improves weight-loss in subjects on a low-calorie diet. (3) Gastrointestinal surgery, which reduces the absorption of the calories ingested, is effective, but has been virtually abandoned because of the side effects it causes. (4) The medicinal approach uses either the anorexigenic action of molecules involved at the level of the central nervous system, or the effect of molecules that increase energy use by increasing the production of heat. The prototypes of this type of molecule are the thyroid hormones that uncouple oxidative phosphorylations of the mitochondrial respiratory chain. The side effects and the toxicity of this type of treatment make their use dangerous. (5) An approach that aims to reduce the absorption of dietary lipids by sequestering them in the lumen of the digestive tube is also in place. However, it induces physiological imbalances which are difficult to tolerate: deficiency in the absorption of fat-soluble vitamins, flatulence and steatorrhoea. Whatever the envisaged therapeutic approach, the treatments of obesity are all characterized by an extremely high recidivation rate.
The molecular mechanisms responsible for obesity in man are complex and involve genetic and environmental factors. Because of the low efficiency of the current treatments, it is urgent to define the genetic mechanisms which determine obesity, so as to be able to develop better targeted medicaments.
More than 20 genes have been studied as possible candidates, either because they have been implicated in diseases of which obesity is one of the clinical manifestations, or because they are homologues of genes involved in obesity in animal models. Situated in the 7q31 chromosomal region, the OB gene is one of the most widely studied. Its product, leptin, is involved in the mechanisms of satiety. Leptin is a plasma protein of 16 kDa produced by adipocytes under the action of various stimuli. Obese mice of the ob/ob type exhibit a deficiency in the leptin gene; this protein is undetectable in the plasma of these animals. The administration of leptin obtained by genetic engineering to ob/ob mice corrects their relative hyperphagia and allows normalization of their weight. This anorexigenic effect of leptin calls into play a receptor of the central nervous system: the ob receptor that belongs to the family of class 1 cytokine receptors. The ob receptor is deficient in obese mice of the db/db strain. The administration of leptin to these mice has no effect on their food intake and does not allow substantial reduction in their weight. The mechanisms by which the ob receptors transmit the signal for satiety are not precisely known. It is possible that neuropeptide Y is involved in this signalling pathway. It is important to specify at this stage that the ob receptors are not the only regulators of appetite. The Melanocortin 4 receptor is also involved since mice made deficient in this receptor are obese (Gura, (1997)).
The discovery of leptin, and the characterization of the leptin receptor at the level of the central nervous system, opened a new route for the search for medicaments against obesity. This model, however, rapidly proved disappointing. With only one exception (Montague et al., (1997)), the genes encoding leptin or its ob receptor, have proved to be normal in obese human subjects. Furthermore and paradoxically, the plasma concentrations of leptin, the satiety hormone, are abnormally high in most obese human subjects.
Clearly there remains a need for novel medicaments that are useful for reducing body weight in humans. Such pharmaceutical compositions advantageously would help to control obesity and thereby alleviate many of the cardiovascular consequences associated with this condition.
The discovery of new genes which are associated to obesity would also allow the design of novel diagnostic and therapeutic tools acting on the lipid metabolism, useful for diagnosing and treating obesity disorders.